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An asteroid the size of a third of a football field passed precariously close to the Earth on Monday.

The proximity of the asteroid's path past the planet had astronomers from NASA and around the world tracking its course and furiously making calculations, said Don Yeomans, manager of NASA's Near Earth Object Program Office. The asteroid is about 30 meters in diameter and weighs in at around 74 million pounds.

At its closest point, it was 41,000 miles from the Earth's surface.

"Celestially speaking, it's quite close," said Yeomans. "It was never a threat to the Earth. On average, something this size hits the Earth about every 500 years — and an object of this size passes this close on average every three to four years. We don't always see them. Sometimes they zip by undiscovered. Our goal is to make sure we see them coming."

Yeomans noted that the last time an asteroid of this approximate size hit the Earth was on June 30, 1908 when a 40-meter asteroid came down in Siberia, leveling trees for 840 square miles. He added that the asteroid exploded about five miles above the Earth, showering the surface with rock pieces.

Astronomers in a NASA-supported observatory in Australia first spotted the approaching asteroid late last week, according to Yeomans. Once they reported their find, other astronomers began keeping track of it. All of the data collected about the asteroid was fed into a program running on a Dell server running eight processors and Unix software. The program, called Sentry, used all the information to automatically make calculations about the asteroid's path, its proximity to Earth and impact probabilities.

In 2005, NASA ran an unmanned spacecraft into a comet that was coming too close to the Earth's surface. Ramming the probe into the comet changed its trajectory, pushing the comet safely away from the planet.

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